The present invention relates to a gas sensor, and more particularly to a gas sensor with a sensor protection element that protects against thermal shock.
The use of gas sensors to monitor oxygen levels in an internal combustion engine is known. Current gas sensors are designed for use in the exhaust manifold, where the sensors detect a lean/rich operating condition of the engine, based on the exhaust gas, and communicate with an engine control unit to manage performance of the engine. The gas sensors include ceramic sensing elements extending from sensor housings for detecting the oxygen levels of the exhaust gas.
At engine start-up, before the exhaust gas heats up to operating temperature, water often exists in the exhaust passage. Because the ceramic sensing elements are heated to very high temperatures for operation, the ceramic sensing elements are susceptible to damage (e.g., cracking) in the event that liquid water contacts the sensing elements. Thus, it may be necessary to delay the operation of the gas sensor until the exhaust gas has heated up sufficiently to eliminate most or all of the liquid water.
One known solution to this problem is disclosed in U.S. patent application Ser. No. 12/130,701, published as U.S. 2009/0101502 and incorporated by reference herein, which provides a plasma-sprayed gamma alumina coating over the sensing element. Although this process has proven to be effective for protecting ceramic sensor elements from thermal shock at temperatures up to about 750 degrees Celsius, the plasma spraying process itself presents inherent cost and risk of sensor damage. Further advances in thermal shock protection and manufacturing cost reductions are desired